When a power relay is used in an application where the normal operating coil voltage is in the range of 24 to 32 volts D.C., but where this voltage frequently drops to as low as 6 to 8 volts D.C, the consequent result would be intermittent opening and chattering of the relay contacts. A simple solution might be to use a relay with a lower voltage coil, but then the excessive heat dissipation created at normal voltage levels would be prohibitive. It has therefore become desirable to combine an electronic circuit with the relay coil system to assure adequate coil power at low voltages but yet limit power at normal voltage levels and to provide such electronic system small enough to enable it to be mounted within the housing of a conventional power relay.
Circuits for controlling power and maintenance coils of an electromagnetic relay have been known heretofore.
One way to do this has been to use dynamic contact switching. For example, C. E. Hayter U.S. Pat. No. 3,108,208, dated Oct. 22, 1963, shows a circuit where, upon application of power, a control relay closes a contact to shunt the auxiliary winding, causing the main winding to be energized across the line. Upon closure, the electromagnetic relay also opens a contact in the control relay circuit to reopen its contact and reinsert the auxiliary winding in series with the main winding for maintaining the relay.
Another known way to do this has been to use a static electronic circuit to control energization of the power and maintenance windings. For example, H. Stamplfi U.S. Pat. No. 3,737,736, dated June 5, 1973, shows a circuit having a full-wave rectified A.C. power source supplying a controllable thyristor that controls energization of the power winding of an electromagnet, the maintenance winding being connected in parallel therewith across the full-wave rectifier bridge, this circuit being limited to use with A.C. since an SCR cannot be turned off in a circuit supplied with D.C.
This invention relates to improvements thereover.